is a Latin M with a diacritical tilde.

Linguistic use

M̃–m̃ is or was used as a grapheme in several languages:
In Italian, the abbreviation Ssm̃o or SSm̃o may be used for Santissimo, "most holy".

Symbolic use

M̃ and m̃ are used in mathematics and physics, see Tilde#Mathematics and Tilde#Physics

Computer use

On the Apple ABC extended keyboard, m̃ is made by typing m and then option-shift n.